This invention relates to an electrical component package comprising a component mounted onto a substrate by a plurality of discrete solder bumps reinforced by a polymeric film. More particularly, this invention relates to an improved method for reinforcing a solder bump interconnection by infiltrating a gap between the component and the substrate with a liquid comprising a polymer precursor material which, upon curing, bonds the component to the substrate to reinforce the interconnection.
A typical electrical component package comprises an integrated circuit chip mounted onto a suitable carrier, which is in turn mounted onto a printed circuit board. The chip carrier has a planar interface that overlies a general planar region of the circuit board. The area of the carrier interface is relatively small in comparison to the total surface of the printed circuit board, so that the carrier attachment region of the circuit board constitutes but a minor portion of the total surface. The carrier interface comprises a plurality of discrete electrical contacts arranged in an array or other suitable pattern. The attachment region of the board also includes a plurality of electrical contacts disposed such that each contact underlies a corresponding contact on the carrier interface and is soldered thereto. The board contacts communicate with electrical signal processing features located on surrounding regions of the board and, through the several solder interconnections, conduct electrical signals to and from the integrated circuit chip for processing.
A convenient technique for mounting the chip carrier to the circuit board utilizes metallic bumps formed from microspheres of a solder alloy. A microsphere is placed onto each contact of the carrier, heated to reflow the solder and cooled, thereby forming a bump bonded to the contact. The carrier with the bumps is positioned on the circuit board region with each bump resting upon a contact on the circuit board. Following heating and cooling, the solder bonds to the circuit board contact, attaching the carrier to the circuit board and electrically connecting the contacts. The carrier interface and the circuit board are spaced apart in the product package by a gap created by the solder bumps.
The circuit board is formed of an epoxy laminant, whereas the chip carrier is composed of alumina or other suitable ceramic material. During thermal cycling such as is typically experienced by the electrical component package during operation, the epoxy laminate board and the ceramic carrier tend to expand and contract, but at different rates. This differential thermal expansion generates stresses within the solder bumps, which produce fatigue within the metal and may lead to failure of the solder bond at the electrical contact. It has been proposed to fill the gap between the electrical components with a polymeric layer in order to reinforce the assembly and reduce stresses on the solder bond during thermal cycling. However, attempts to infiltrate a solder bump interconnection on a printed circuit board with a polymer precursor liquid have not heretofore been entirely satisfactory because of the difficulty of applying the liquid to flow into and fill the gap without indiscriminately flowing across the circuit board surface.
It is an object of this invention to provide an electrical component package comprising an electrical component, such as a semiconductor chip carrier, assembled to a selected region of a substrate, such as a printed circuit board, by polymer-reinforced solder bump interconnection, wherein the reinforcing polymer film completely fills the gap between the electrical component and the printed circuit board region while being confined to said region by a preformed polymeric dam.
It is a further object of this invention to provide an improved method for assembling an electrical component to a substrate by a polymer-reinforced solder bump interconnection, wherein, following attachment of the component to the substrate by a plurality of solder bumps, a pool of polymer precursor liquid is flowed onto a region of the substrate immediately about the attached component and is confined by a dam to fill the gap created by the solder bumps. Upon curing, the resulting polymeric film reinforces the solder bump interconnection. In certain aspects of this invention wherein the polymer liquid is first applied about the component and flows into the gap, gas is evacuated or vented from the gap to promote the flow of liquid thereinto. In another aspect of this invention, polymeric liquid is injected into the gap through a passage extending through the substrate and flows outwardly thereabout to form the desired reinforcing film.